This invention relates to a liquid crystal display device (LCD) of the transmission type on which a light interrupting black mask layer is formed.
Conventionally, a liquid crystal display device of the transmission negative display type is known which is covered, except a display pattern thereof, with a mask layer to interrupt transmission of light through a background area thereof. One of such mask layers for interrupting transmission of light is a black mask layer which is formed by application of black ink using a technique of offset printing or the like.
FIG. 1 shows an exemplary one of such liquid crystal display devices having such a black mask layer formed thereon. Referring to FIG. 1, the liquid crystal display device shown in generally denoted at 1 and includes a liquid crystal cell 2, a pair of polarizing plates 3 applied to opposite outer faces of the liquid crystal cell 2, and a color filter 4 applied to one of the polarizing plates 3. The liquid crystal cell 2 includes an upper glass substrate 5, a lower glass substrate 6, and a liquid crystal layer 7 formed between the upper and lower glass substrates 5, 6 and enclosed airtight by means of sealing elements not shown. A common electrode 9 is formed on an inner face of the upper glass substrate 5 via an undercoat 8 made of silicon carbide, and an orientation film 10 is formed on an inner face of the common electrode 9. Meanwhile, a plurality of segment electrodes 12 constituting a display pattern are formed on an inner face of the lower glass substrate 6 via another undercoat 11 made of silicon carbide. A plurality of black mask layers 13 constituting a background area are formed at locations of the undercoat 11 other than those locations on which the segment electrodes 12 are formed. Here, the black mask layers 13 are formed by application of black ink by offset printing or the like as described above. Further, another orientation film 14 is formed on the undercoat 11 and covers the segments electrodes 12 and the black mask layers 13. Liquid crystal is filled between the orientation film 10 of the upper glass substrate 5 and the other orientation film 14 of the lower glass substrate 6 to form the liquid crystal layer 7.
In the liquid crystal cell 2, the polarizing plates 3 are applied to the outer faces of the upper glass substrate 5 and the lower glass substrate 6 such that polarization axes thereof may coincide with each other, and the color film 4 is applied to an outer face of one of the polarizing plates 3 adjacent the lower glass substrate 6.
By the way, it is a problem that, when liquid crystal display devices of such a construction as described above are produced, the ratio of rejects due to electric connection between a common electrode and a segment electrode is relatively high and accordingly the yield upon production is low. The inventors have made several investigations of a cause of this and found out that it is a principal cause that, because particles of carbon in the back ink forming a black mask layer normally have a diameter of 0.4 to 0.8 microns and are thus relatively large and non-uniform, they are apt to aggregate themselves and such aggregations of carbon particles on the black mask layers 13 may make a projection 15 as shown in FIG. 2 which extends to the common electrode 9 to short-circuit the common electrode 9 and a segment electrode 12.
Here, it may seem recommendable, as means for removing such projections 15 caused by aggregation of carbon particles, to roll surfaces of the black mask layers 13 by means of a roll or the like around which a silicone rubber sheet is wrapped so as to crush such projections 15. However, it is difficult to crush such projections 15 because carbon is hard compared to silicone rubber. Therefore, such crushing is insufficient to remove the projections 15 to prevent electric connection between the common electrode 9 and a segment electrode 12.